Aircraft flight controls that have power actuators for controlling the position of the control surfaces, such as electric motors and hydraulic or pneumatic actuators, lack the ability to transmit control surface forces to the pilot through the control stick or rudder pedals. The pilot thus does not have an indication of a so-called "load feel" which would indicate control surface forces. It therefore is common practice in arrangements of this type to provide a synthetic feel in order to simulate conventional control forces acting on the control surfaces of an aircraft during flight maneuvers. The presence of a so-called control feel is especially important during control of aircraft elevator surfaces where acceleration forces of the aircraft in the direction of the vertical axis of the aircraft might have a potential for causing a structural overload.
It is known in the prior art to provide an artificial load feel for the pilot by using springs and bobweights. The bobweight makes a correction for stick force by providing an increment in the normal acceleration force equal to the acceleration of gravity or "g" loading. Such a correction will improve the pilot's perception of maneuvering stability of the aircraft. Another desirable characteristic of a pilot-operated control surface actuator is the ability of the forces acting on the pilot-operated element to return the pilot-operated element to a neutral position quickly.
It is desirable for the artificial feel that is developed by the powered control surface mechanism to be approximately proportional to the acceleration forces induced by the pilot in his or her control action. The optimum feedback force required for improved control effectiveness at high airspeeds is higher than the force required for low airspeeds. Further, it is necessary that artificial feedback mechanisms provide force gradients that are sufficient to ensure stability of the aircraft. That is, it is necessary for effective stability control that the pilot input force be generally proportional to incremental acceleration.
I am aware of prior art designs in which control surface feedback forces generated by artificial feel systems are varied in response to changes in airspeed and other operating variables, including altitude changes and changes in the center of gravity of the aircraft during flight. U.S. Pat. No. 3,747,876, for example, shows force gradients that may be varied using a leverage system that is subjected to spring forces wherein the mechanical advantage of the leverage system can be changed by an actuator that responds to changes in airspeed. The system of the '876 patent, however, does not employ a simplified bobweight mechanism. Rather, it uses a multiple lever actuator and cam actuator system with a means for varying the leverage ratio of the control surface actuator linkage.
Prior art U.S. Pat. No. 2,661,169 shows an aircraft control stick force linkage with a stationary bobweight that creates a force moment on the control stick of an aircraft. It is balanced by a balance spring in a conventional fashion. The control stick leverage system, however, has an auxiliary linkage that extends to a force bellows, the opposite sides of which are subjected to the static pressure on the upper surface of an airfoil and to the ram pressure at the lower surface. Thus, the bellows force is a function of the square of the aircraft airspeed. This bellows force is transmitted to the aircraft control stick to complement the force moment imparted to the control stick by the bobweight. The bobweight force, as in the case of other known bobweight designs, cannot be varied in response to changing operating variables that are experienced during maneuver of the aircraft.